1. Field of Invention
The present invention relates to a method for fabricating a read-only memory (ROM). More particularly, the present invention relates to a method for fabricating a nitride read-only memory.
2. Description of Related Art
A method for fabricating a nitride read-only memory in the prior art is described as follows. An oxide/nitride/oxide (ONO) stacked charge-trapping layer is formed on a substrate, and then patterned into a plurality of ONO strips. An ion implantation is performed to form doped regions in the exposed substrate serving as buried bit lines. An isolating oxide layer is formed on each doped region, and then a plurality of polysilicon word lines are formed on the substrate crossing over the ONO strips.
In the prior art, the ONO stacked layer is patterned with a patterned photoresist layer as an etching mask. However, as shown in FIG. 1, when the bottom oxide layer of the ONO stacked layer 108 is being etched, the top oxide layer 106 is etched simultaneously to cause corner loss or even corner indents. Therefore, the electrical properties around the edges of the ONO stacked layer 108 are changed, which may result in severe problems.
To prevent the problems caused by corner loss or corner indents, a method is provided in the prior art that increases the thickness of the top oxide layer to reduce the degree of the influence of corner loss or corner indents. However, an increased thickness of the top oxide layer does not meet the requirements for device miniaturization.